This invention relates to controlling the release of the brake on a drill head, and particularly to a method and apparatus which ensures that the brake will not be released unless sufficient hydraulic pressure is present to hold the drill head from falling.
Blast hole drills typically include a drill head mounted on a drill mast. The drill head raises and lowers a drill string and also rotates the drill string. An example of a blast hole drill of this type is described in U.S. Pat. No. 4,793,421, issued Dec. 27, 1988, for "Programmed Automatic Drill Control".
The drill head may be powered with electric or hydraulic motors for the rotary drive. The drill head may be hoisted and pulled down by electric or hydraulic motors or by hydraulic cylinders. If hydraulic, the system may consist of an open or closed circuit. The present invention is adapted for hydraulic systems.
The drill head contains a brake that holds it in place on the drill mast. The brake may consist of spring loaded calipers which normally engage or set the brake. When the head brake is released, the weight of the head is supported on the column of hydraulic fluid in a hoist line leading to the drill head motor. With the brake set, after the blast hole drill is shut down, the possibility exists that there will be insufficient pressure in the hoist line to support the head once the brake is released.